The present invention relates in general to a digital radio receiver employing quadrature signal processing and more specifically to a digital radio receiver for reproducing AM stereo radio signals.
Commercial radio services, such as AM and FM, are broadcast as a modulated analog signal. Certain radio receivers have been employed which convert the received analog signal into a digital signal for performing digital signal processing (DSP) to thereby realize various advantages, such as circuit integration, reduced size, exact operation, minimal adjustments, and the ability to combine signal processing with other audio functions (e.g., tone control, concert hall emulation, and equalization) also performed in DSP.
The cost of implementing a particular receiver using DSP components increases with the amount of processing required to perform the desired functions. The processing load of a particular DSP system depends on the functions implemented and on the number of samples in the digital signal which must be processed in a given amount of time, i.e., the sampling rate f.sub.s in samples per second. The sampling rate f.sub.s must be at least two times the frequency bandwidth of the sampled signal to avoid distortion. In a radio receiver, the sampled signal is typically an intermediate frequency (IF) signal from an analog tuner stage. The analog IF is centered at a nonzero frequency and is typically 455 kHz in an AM receiver and 10.7 MHz in an FM receiver.
In order to initiate digital signal processing, the IF signal is converted to a digital IF signal using an analog-to-digital (A/D) converter.
It is known to reduce digital processing load by representing the digital IF signal as a quadrature signal with an in-phase (I) component and a quadrature-phase (Q) component. It is preferable to form the I and Q signals after conversion to digital signals due to the difficulties encountered in matching the I and Q signal path characteristics in analog circuitry.
U.S. Pat. No. 4,893,316 to Janc et al discloses a digital signal processing receiver employing quadrature signals. Janc et al employs two injection (i.e., mixing) signals from a digital quadrature local oscillator for mixing with the digital signal to produce the I and Q signal components comprising a nominal zero-Hertz intermediate frequency.
U.S. Pat. No. 4,592,074 to Whikehart teaches a technique for forming I and Q components simultaneously with a reduction in sampling rate in a digital filter without the need for actual injection signals. By selecting the input IF frequency and the injection frequency to both be at one-quarter of the sampling rate f.sub.s, the values for the injection signal become repeating sine and cosine sequences of the values 1, 0, and -1, which can be directly implemented in the digital filter without an oscillator.
The technique taught in U.S. Pat. No. 4,592,074 requires that the injection frequency remain fixed at one-quarter of the sampling rate. However, depending on the particular radio signal to be received, it may be necessary or desirable to employ a signal detection scheme using a variable injection frequency. One such particular type of radio signal is an AM stereo signal which includes phase-encoded information that must be demodulated using synchronous detection. Synchronous detection requires a changing injection frequency locked to the frequency of the IF signal in order to recover the original audio signals without distortion.